Radial piston units find several applications in fluid power, offering benefits of low noise and high power density. The capability to generate high pressures makes radial piston pumps suitable for clamping function in machine tools and also to operate presses for sheet metal forming.This study is aimed at developing a comprehensive multidomain simulation tool to model the operation of a rotating cam type radial piston pump, with particular reference to the lubricating gap flow between the pistons and the cylinder block. The model consists of a first module which simulates the main flow through the unit according to a lumped parameter approach.This module evaluates the features of the displacing action accounting for the detailed evaluation of the machine kinematics and for the mechanical dynamics of the check valves used to control the timing for the connection of each piston chamber with the inlet and outlet port. The estimation of the instantaneous pressure within each displacement chamber is utilized in a second module dedicated to the prediction of the lubricating gap flow between the pistons and the cylinder. This lubricating gap flow module is based on a fluid-structure interaction algorithm in which the fluid film pressure is calculated by using the Reynolds equation and accounting for hydrodynamic terms due to material deformation and micro-motions of the pistons.The results shown in the paper highlight the potentials of the simulation approach for determining the main performance parameters of the unit (i.e. outlet flow oscillations, instantaneous piston pressures, etc.) including the evaluation of possible contact and of the power losses due to leakages or fluid shear at the piston/cylinder lubricating interfaces.